Electrophotographic copying is a process where an electrostatic latent image is produced by a strong light flash. A colored, finely-divided powder, a dry toner, is used to develop and transfer this latent image into a visible image, generally on standard, plain paper. See Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed. Vol. 8, 1978, pp. 794-826, which is incorporated herein by reference. See also "Pigment Blacks in Toners for Electrophotographic Copying Process", American Ink Maker, July 1987, pp. 18-20, which is incorporated herein by reference.
Although there are a variety of machines with different toner requirements, some of the steps in electrophotography are common to almost all systems. The first step is electrostatic charging or sensitization of the semiconductor photoconductor layer through corona discharge. Then, a latent electrostatic image of the original item to be copied is generated on this photoreceptor surface by exposure to light. This latent image corresponds to the intensity of light and dark areas in the original. This electrostatic image is developed by contact with a toner having an opposite triboelectric charge. The toner particles are then transferred from the semiconductor layer to the image carrier, e.g., the paper. The image is finally fixed, generally by means of heat and/or pressure. Lastly, the photoreceptor is cleaned of toner and neutralized of any residual charge.
A variety of materials have been used as toners in copiers. These are described in the Encyclopedia of Materials Science and Engineering Vol. 2, pp. 1503-1508, 1986, which is incorporated herein by reference. Typically, thermoplastic organic polymers are used along with a colorant and a charge agent. Copolymers of acrylates or methacrylates and styrenes are the primary thermoplastics used in most commercial toners. Charge agents include such diverse components as ammonium salts and dyes. The natural mineral uintaite has been used in toner compositions.
Desirable toner properties include low cost, hardness, friability, hydrophobility, good adhesion to paper, and a gloss transition temperature of about 60.degree. C. Rheology and charge characteristics must be consistent with the fixing method.
Uintaite is a naturally occurring hydrocarbon mineral. As a natural product, the chemical and physical properties of uintaite vary and depend strongly on its source. Uintaite has also been called "gilsonite" although this usage is incorrect; Gilsonite.RTM. is a registered trademark of American Gilsonite Co., Salt Lake City, Utah. American Gilsonite Co. sells a variety of Gilsonite.RTM. "resins", sometimes identified by their softening points (ring and ball). For example, General Purpose (GP) Gilsonite.RTM. brand resin has a softening point of about 350.degree. F. Higher valued grades of uintaite have lower softening points: these include Gilsonite.RTM. "Selects", for which the lowest softening point is about 290.degree. F. The softening points of these natural uintaites depend primarily on the source vein that is mined when the mineral is produced. Uintaite is described in the Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed. Vol. 11, 1980, pp. 802-806, which is incorporated herein by reference.
Uintaite is a unique material. It is primarily hydrocarbon in nature, is very shiny and hard, and has a high melt viscosity. Although it is often compared to asphalt, many of its properties clearly differentiate it.
Uintaite has been separated into fractions. For example, sulfuric acid has been used to extract nitrogen containing compounds from uintaite. U.S. Pat. No. 3,414,128 to Baarson et al., describes the use of nitrogenous materials obtained from uintaite in froth flotation. This patent also discloses fractions of uintaite prepared by thermal cracking and distillation.
Uintaite has also been fractionated by solvent extraction. It is believed that samples of these fractions have been available since the 1950's.
Also, bituminous materials, such as asphalts and uintaite, have been used as "extractants" for mixtures containing aliphatic and aromatic hydrocarbon; see U.S. Pat. No. 3,190,934 to Garwin.
As already stated, uintaite has been used in toner compositions. One example of a toner containing uintaite is U.S. Pat. No. 3,652,315 to Matsuo et al., which discloses the use of uintaite and carbon black as coloring agents in compositions where toner particles are coated with liquids, such as silicone oil. Another example is U.S. Pat. No. 3,671,646 to Kurita et al., which discloses in Example 4 a liquid developer for electrostatic photography that includes natural uintaite. Still another toner composition that includes natural uintaite is described in U.S. Pat. No. 4,254,201 to Sawai et al. The uintaite is described as a pressure sensitive adhesive substance, a tackifier.
Natural uintaite has also been used in printing inks. Japanese Patent J60071675A to Moroboshi Ink KK describes a low viscosity lithographic printing ink prepared by dissolving uintaite is an oil. U.S. Pat. No. 3,427,967 to Thielen describes low condensate inks comprising high grade uintaite. U.S. Pat. No. 4,773,932 to Gambling teaches a printing ink comprising pigment, oil, tall oil pitch and optionally, gilsonite. This patent teaches that these compositions have reduced rub off relative to conventional inks.
U.S. Pat. No. 4,193,815 to Burris teaches gilsonite (uintaite) emulsion compositions containing 33-95% gilsonite together with a petroleum hydrocarbon of boiling point above about 450.degree. F. (such as diesel oil, gas oil, lubricating oil and asphalt) and a water phase consisting essentially of water and a small amount of emulsifying agent. The emulsifying agents taught, include, among others, the alkali metal salts of higher fatty acids, or mixtures of acids available from animal or vegetable oils. The amount of emulsifier is between 0.05 and about 5% of the total composition weight. These compositions are described as useful for producing insulation board and for printing inks. U.S. Pat. No. 4,282,037, also to Burris, teaches similar compositions.
Additionally, Japanese patent 80 018468-B teaches a printing ink prepared by emulsifying an aqueous solution containing an inorganic acid or its salt with a nonionic surfactant of 8-18 HLB with a vehicle formed by dissolving one or more resins selected from petroleum resin, hardened rosin, ester gum, tall oil pitch and gilsonite in mineral oil.
All of the above patent teach compositions comprising natural unitaite. However, a number of problems are observed when uintaite is used in toners or inks. These problems are related to the physical and chemical properties of uintaite. For example, uintaite has a very high softening point and very high melt and solution viscosities. These properties make it difficult to use uintaite in the manufacture of toners and inks using standard techniques. Also, uintaite as mined generally contains naturally occurring mineral insolubles which can be problematic during processing and use, especially in inks where the uintaite is used in solution. Furthermore, the viscosity of uintaite in solution increases over time. This viscosity instability makes handling uintaite in solution difficult, and prevents its use in applications where solutions are stored for periods of time.
Moreover, the odor of uintaite has been found objectionable; and its compatibility with and solubility in organic solvents is limited.
A very significant problem with uintaite can be the variability of its properties. Since uintaite is a natural product, its physical and chemical properties depend strongly on its source. Thus, its use, for example in electrophotography, requires constant experimentation and monitoring which significantly reduces its effectiveness and applicability in most conventional copy systems. As the source varies, the properties of the uintaite vary.
It would be advantageous if a composition could be provided that would have the desirable properties of uintaite, for example, its shine, that would not have the disadvantages of uintaite, for example, the undesirably high melt viscosity and hardness, and the viscosity instability in solution, that would be useful in toners.
It would be advantageous if a uintaite-derived composition with predetermined properties could be provided that in combination with a thermoplastic resin, a colorant or both, would be useful for electrophotography or for printing.
It would be advantageous if a single material could be provided that could act as a thermoplastic resin, as a positive charge agent and as a colorant in toner compositions.
Among other factors, it has unexpectedly been found that purified fractions of uintaite can partially replace synthetic polymers in toner compositions, while also providing sufficient charge so that charge agents are not necessary. One object of the present invention is to provide a composition that is useful in electrophotography. Another object of the present invention is to provide compositions useful in printing. Further objects of the invention will become evident to those skilled in the art.